Braer storm satellite image and animation

 

 

25/10/2008 around 14.30.  Probably gusting to storm force 11. Nothing special for Eshaness but a fine video from shetlandic.

 

Wind extremes

The frequency of Atlantic depressions passing Shetland gives average wind speeds that are noticeably higher than at low level stations on mainland U.K. Lerwick Observatory experiences 42 gale days a year, with no month free of gales. January is the windiest month with an average of 8 gale days.

January 1993 was of exceptional significance, not just for the wreck of the oil tanker Braer, but for the phenomenal incidence of gales that generated the heavy seas which cleaned up much of the oil spill. There were 25 gale days recorded for the month of which 10 were storm force, 10 or more with 18 consecutive gale days from the 1st to the 18th. This extreme month also set the highest hourly mean wind recorded at Lerwick Observatory (66kt).

Fast-moving and rapidly-developing depressions are the main cause of severe winds in Shetland of which the ‘New Year Hurricane’ of 1992 is an extreme example. This event on 1 January 1992 led to widespread damage and caused two deaths by blowing a hut and its occupants over the cliffs at Hermaness, Unst.

At midday on 31 December 1991 a low pressure centre of 985mb had developed at the left exit of a strong WSW jet at 57N 27W. At this time a very sharp thermal trough extended from SW Iceland to the Hebrides with a thermal ridge building behind it. The Positive Vorticity Advection Area associated with the development soon caught up with the strong warm advection driving the thermal ridge. This caused explosive cyclogenesis and deepened the low to 966mb by 1800.

At midnight the left exit of the jet was just behind the top of a sharp thermal ridge just west of Faeroe, deepening the low to 957mb just NW of Faeroe. The centre had travelled 660 nautical miles in 12 hours (a mean speed of 55kt). The low continued to deepen as it passed north of Faeroe falling to 947mb by 0600 on the 1st of January.

Pressure falls were in the order of 7mb/hr across Faeroe and 5mb/hr over Shetland. Muckle Flugga Lighthouse recorded all time record wind speeds for a low level station in the British Isles; mean speed 73kt (89mph), with gusts over the instrument maximum of l50kt (173mph). The instrument was destroyed by a severe squall after 0300, probably as the wind veered on the passage of the cold front through the station. An unofficial record from an oil rig NE of Unst placed mean speeds at l03kt (125mph), with gusts at 169kt (194mph).

Allen Fraser